Irradiated multilayer film having seal layer containing hyperbranched polymer

ABSTRACT

An irradiated multilayer film has a first layer and a second layer. The first layer is an outer heat seal layer containing a blend of a highly branched homogeneous polymer and a semicrystalline polymer. The highly branched homogeneous polymer has at least 40 branches per 1000 methylene groups, an M w /M n  less than 3.0, and a density of less than 0.89 g/cc. The highly branched homogeneous polymer is present in an amount of from about 5 to 50 weight percent, based on layer weight. The semicrystalline polyolefin has a density of at least 0.90 g/cc. The semicrystalline polyolefin is present in an amount of from about 50 to 95 weight percent, based on total layer weight. The second layer contains a thermoplastic polymer in a crosslinked polymer network. A process for making a packaging article includes coextruding the first and second layers, irradiating the resulting multilayer film, and thereafter heat sealing the first layer to itself or another component of the packaging article.

This application claims the benefit of provisional application no.60/333,285 filed Nov. 6, 2001.

FIELD OF THE INVENTION

The present invention pertains to films which are to be heat sealed,especially flexible packaging films.

BACKGROUND OF THE INVENTION

Packaging films are frequently heat sealed in order that the film isconverted to a packaging article, such as a bag, pouch, backseamedcasing, etc. Irradiation of the polymer in the film, such as E-beamirradiation, crosslinks the polymer in the film, making it stronger. Ifthe seal layer is irradiated, the crosslinking of the polymer can resultin a seal layer which has diminished flow characteristics. However, ifit is desired to irradiate other layers of the film, it may also benecessary to irradiate the seal layer. It would be desirable to providea film which can be made stronger by irradiation, without substantiallydiminishing the heat seal performance of the seal layer.

SUMMARY OF THE INVENTION

The present invention is directed to an irradiated multilayer filmhaving a seal layer containing a blend of a highly branched homogeneouspolymer with a semicrystalline polymer. This blend can be irradiatedwithout substantially diminishing the heat seal performance of the seallayer. The highly branched homogeneous polymer can be subjected toirradiation without resulting in a crosslinked structure whichdiminished the heat seal performance of the seal layer. The highlybranched homogeneous polymer also prevents the semicrystalline polymerfrom crosslinking in a manner which substantially reduces heat sealperformance.

As a first aspect, the present invention is directed to an irradiatedmultilayer film has a heat seal layer containing a blend of a highlybranched homogeneous polymer and a semicrystalline polyolefin. Thehighly branched homogeneous polymer has at least 40 branches per 1000methylene groups, has a M_(w)/M_(n) less than 3.0, and a density of lessthan 0.89 g/cc. The highly branched homogeneous polymer is present in anamount of from about 5 to 50 weight percent (preferably, 20 to 40 weightpercent) based on layer weight. The semicrystalline polyolefin has adensity of at least 0.90 g/cc, and is present in an amount of from about50 to 95 weight percent (preferably, from 60 to 80 weight percent),based on total layer weight. The second layer contains a crosslinkedpolymer network. Preferably, the heat seal has a seal strength at 75° C.of at least 100 grams per centimeter. The seal strength is preferablyfrom 100 to 2000 grams per centimeter, more preferably from 200 to 1000grams per centimeter. Preferably, the seal has a strength of at least200 grams per centimeter.

Preferably, the semicrystalline polyolefin has a density of at least0.905 g/cc, more preferably at least 0.91 g/cc, and can even have adensity of at least 0.92 g/cc.

Preferably, the highly branched homogeneous polymer has from 40 to 120branches per 1000 methylene groups, more preferably, from 40 to 90branches per 1000 methylene groups, and still more preferably, from 45to 80 branches per 1000 methylene groups.

Preferably, the semicrystalline polyolefin comprises at least one memberselected from the group consisting of very low density polyethylene,linear low density polyethylene, low density polyethylene, high densitypolyethylene, propylene homopolymer, propylene copolymer, linearhomogeneous ethylene/alpha-olefin copolymer, homogeneousethylene/alpha-olefin copolymer having long chain branching. Thesemicrystalline polyolefin is different from the highly branchedhomogeneous polymer in that the semicrystalline polymer, if branched,has less than 60 branches per 1000 methylene groups. Preferably, thehighly branched homogeneous polymer is a homopolymer of ethylene.

In one embodiment, the multilayer film comprises an oxygen barrierlayer. Preferably, such a multilayer film further comprises a tie layerbetween the oxygen barrier layer and the seal layer. Preferably, theheat seal layer is an inside layer of the packaging article, and the tielayer between the oxygen barrier layer and the seal layer is a first tielayer, with the multilayer film further comprising a second tie layerbetween the oxygen barrier layer and an outside layer.

In one embodiment, the film is heat-shrinkable. Preferably, such a filmhas a total free shrink, at 185° F., of at least 20 percent; preferably,from 20 to 120 percent; more preferably, from 20 to 80 percent. Inanother embodiment, the multilayer film has a total free shrink, at 185°F., of less than 10 percent.

Preferably, the film has a thickness of from about 0.3-15 mils; morepreferably, from 0.5 to 10 mils; or 1.5 to 6 mils, or 1.5 to 5 mils, or1.5 to 4 mils, or 2 to 4 mils.

As a second aspect, the present invention is directed to a process formaking a packaging article. The process comprises coextruding amultilayer film comprising a first layer which is a heat seal layer, anda second layer containing a thermoplastic polymer. After coextrusion ofthe first and second layers, the resulting multilayer film is irradiatedto crosslink the thermoplastic polymer of the second layer. The heatseal layer is then sealed to itself or another component of thepackaging article. The heat seal layer is in accordance with the firstaspect of the present invention. Preferably, the heat seal has a sealstrength of at least 75 grams per centimeter.

DETAILED DESCRIPTION OF THE INVENTION

The multilayer films of the present invention are preferably irradiatedto induce crosslinking, as well as corona treated to roughen the surfaceof the films which are to be adhered to one another. In the irradiationprocess, the film is subjected to an energetic radiation treatment, suchas corona discharge, plasma, flame, ultraviolet, X-ray, gamma ray, betaray, and high energy electron treatment, which induce cross-linkingbetween molecules of the irradiated material. The irradiation ofpolymeric films is disclosed in U.S. Pat. No. 4,064,296, to BORNSTEIN,et. A1., which is hereby incorporated in its entirety, by referencethereto. BORNSTEIN, et. A1. Discloses the use of ionizing radiation forcrosslinking the polymer present in the film.

Radiation dosages are referred to herein in terms of the radiation unit“RAD”, with one million RADS, also known as a megarad, being designatedas “MR”, or, in terms of the radiation unit kiloGray (kGy), with 10kiloGray representing 1 MR, as is known to those of skill in the art. Asuitable radiation dosage of high energy electrons is in the range of upto about 16 to 166 kGy, more preferably about 40 to 90 kGy, and stillmore preferably, 55 to 75 kGy. Preferably, irradiation is carried out byan electron accelerator and the dosage level is determined by standarddosimetry processes. Other accelerators such as a van der Graaf orresonating transformer may be used. The radiation is not limited toelectrons from an accelerator since any ionizing radiation may be used.

As used herein, the phrases “corona treatment” and “corona dischargetreatment” refer to subjecting the surfaces of thermoplastic materials,such as polyolefins, to corona discharge, i.e., the ionization of a gassuch as air in close proximity to a film surface, the ionizationinitiated by a high voltage passed through a nearby electrode, andcausing oxidation and other changes to the film surface, such as surfaceroughness.

Corona treatment of polymeric materials is disclosed in U.S. Pat. No.4,120,716, to BONET, issued Oct. 17, 1978, herein incorporated in itsentirety by reference thereto, discloses improved adherencecharacteristics of the surface of polyethylene by corona treatment, tooxidize the polyethylene surface. U.S. Pat. No. 4,879,430, to HOFFMAN,also hereby incorporated in its entirety by reference thereto, disclosesthe use of corona discharge for the treatment of plastic webs for use inmeat cook-in packaging, with the corona treatment of the inside surfaceof the web to increase the adhesion of the meat to the adhesion of themeat to the proteinaceous material. Although corona treatment is apreferred treatment of the multilayer films used to make the heat-sealedarticle and packaged product of the present invention, plasma treatmentof the film may also be used.

Preparation of the highly branched homogeneous polymer (also sometimesreferred to by those of skill in the art as a “hyperbranched homogeneouspolymer”) used in the present invention is described in U.S. Pat. No.5,880,241, to Brookhart et al, entitled “Olefin Polymers”, herebyincorporated in its entirety, by reference thereto. The polymers ofBrookhart et al are disclosed as having a unique structure in terms ofthe branching in the polymer, with the number of branches, and to acertain extent the length of the branches, being determined by NMRspectroscopy. The amount of branching is expressed in terms of thenumber of branches per 1000 of the total methylene (—CH₂—) groups in thepolymer, with one exception. Methylene groups that are in an estergrouping, i.e. —CO₂ R, are not counted as part of the 1000 methylenes.Otherwise, the methylene groups include those in the main chain and inthe branches.

As used herein, the phrase “highly branched homogeneous polymer” (i.e.,“HBH polymer”) refers to single site catalyzed resin with a polymerarchitecture wherein there are at least 40 side branches from the mainchain for every 1000 main chain carbons. While not wishing to berestricted to any single sub-architecture, it is possible that some ofthe side branches may themselves contain side branches, similar todendritic, or “tree-like” structures. It is believed from observationsof the performance of these highly branched homogeneous polymers, thatthe number and type of branching results in a macro-structure whichassumes a somewhat spherical conformation. This substantially sphericaltopology of the polymer chain is believed to be responsible for uniqueflow properties, as a consequence of interaction with other polymerchains.

The HBH polymer useful in the present invention have greater than 40side chain branches per 1000 carbon atoms, preferably at least 50 sidechain branches per 1000 carbon atoms; preferably from about 60 to 140side chain branches per 1000 carbon atoms. For every 100 side chainbranches that are methyl, the HBH polymer has 1 to about 80 ethylbranches, 1 to about 20 propyl branches, 1 to about 50 butyl branches, 1to about 20 amyl branches, and 1 to about 100 hexyl or longer branches.

More preferably, the HBH polyolefins useful in the present inventionhave from about 40 to 110 side chain branches per 1000 carbon atoms. Forevery methyl 100 side chain branches, preferably there are from 1 toabout 20 ethyl branches, from 1 to about 10 propyl branches, from 1 toabout 15 butyl branches, from 1 to about 10 amyl branches, and from 1 toabout 30 hexyl or longer branches.

More preferably, the HBH polymer useful in the present invention havefrom about 50 to 100 side chain branches per 1000 carbon atoms. Forevery 100 branches that are methyl, preferably there are from about 2 toabout 18 ethyl branches, from about 2 to about 8 propyl branches, fromabout 2 to about 12 butyl branches, from 2 to about 8 amyl branches, andfrom about 8 to about 30 hexyl or longer branches.

The HBH polymer is preferably a narrow molecular weight (Mw/Mn), singlesite catalyzed resins. The HBH polymer preferably has a molecular weightdistribution less than 3, preferably less than 2.5. However, it ispossible to prepare HBH polymer having greater Mw/Mn using tandemreactor processes which can result in bimodal or multimodal productscomprising the HBH polyolefins.

The HBH polymer exhibits a melt index of from about 0.5 to about 10 g/10min, preferably from about 1 to 9, more preferably from about 1.1 to8.5, more preferably from about 1.5 to about 7.5. The HBH polyethyleneuseful in the present invention has a molecular weight (Mw) of fromabout 80,000 to about 200,000, preferably from about 90,000 to about150,000. The HBH polymer may be prepared by methods of synthesisdisclosed herein, preferably using nickel (II) α-diimine catalystcomplexes. Other methods of preparing HBH polyolefin include methodsdisclosed in U.S. Pat. No. 5,866,663 to Brookhart et al. entitled“Process of Polymerizing Olefins”, hereby incorporated in its entirety,by reference thereto.

The HBH polymer useful in the present invention can alternatively beevaluated via proton NMR or ¹³C NMR. Structural assignments for thepolymers of the invention were determined by ¹³C NMR. Methodology andinstrumental conditions used were in accord with ASTM method D-5017-96.Approximately 150 mg of polymer sample, 2 ml 1,2,4-trichlorobenzene,0.75 ml of benzene-d6 (external lock), and 0.25 ml ofhexamethyldisiloxane (internal reference) were loaded into a 10 mm NMRsample tube. The contents of the sample tube were homogenized using aheating block at 130° C. for 3-4 hours. NMR data were collected at 130°C. using a Bruker DMX-400 instrument (¹³C resonance at 50.3 MHz). Thepulse width was 90° as measured using the resonance of the isolatedmethylenes at 30.0 ppm. 3000 scans were collected for each specimen (atleast 32,000 data points) at a 150 ppm sweep width. The pulse repetitionrate used was 10 seconds with complete proton decoupling. All ¹³C-NMRsignals from 50-10 ppm were integrated. Total side chain branching wascalculated from integral ratios of methyl groups at branch ends to totalpolymeric ¹³C-NMR signal (methyls per 1000 carbon atoms). Branchdistribution was calculated from integral ratios of specific branchlength (type) resonances. Signals from side branches of six carbons andlonger are unresolved and are therefore integrated as one compositesignal (C6+).

HBH polyolefin has at least 50 branches per 1000 carbon atoms,preferably at least 60 branches per 1000 carbon atoms, in which at least50 percent of the branches are methyl, from 2 to about 25 percent of thebranches are ethyl, from 1 to about 20 percent of the branches arepropyl, and from 1 to about 20 percent of the branches are butyl, andfrom 1 to about 20 percent of the branches are amyl, and from about 5 toabout 50 percent of the branches are hexyl or longer. Preferably, theHBH polymer has at least 60 branches per 1000 carbon atoms, in which atleast 60 to about 80 percent of the branches are methyl, from about 4 toabout 20 percent of the branches are ethyl, from about 2 to about 15percent of the branches are propyl, and from about 2 to about 20 percentof the branches are butyl, and from about 2 to about 15 percent of thebranches are amyl, and from about 5 to about 30 percent of the branchesare hexyl or longer.

More preferably, the HBH polyolefin has at least 70 branches per 1000carbon atoms, in which at least 70 percent of the branches are methyl,from about 2 to about 10 percent of the branches are ethyl, from about 2to about 10 percent of the branches are propyl, and from about 2 toabout 10 percent of the branches are butyl, and from about 2 to about 10percent of the branches are amyl, and from about 7 to about 20 percentof the branches are hexyl or longer.

Preferably, the HBH polymer in the film of the invention is a HBHpolyolefin, preferably a HBH polyethylene. In one preferred embodiment,at least one outer layer of the film contains one or more HBHpolyethylene, which may make up 100 percent of the weight of the filmlayer. Alternatively, the HBH polymer is blended with one or moreadditional polymers and/or additives (such a slip agents, antiblockagents, etc). If another polymer is present, the HBH polymer preferablycomprises at least 20% of the weight of the layer. Preferably, the HBHpolyolefin comprises about 30% by weight of the blend. More preferably,the HBH polyolefin comprises about 40% by weight of the blend. Preferredranges for resin blends comprising the HBH polyolefin are in the rangeof from about 10 percent to about 90 percent, preferably about 20 toabout 80 percent, preferably from about 30 percent to about 75 percent,still more preferably from about 40 to 70 percent of the HBH polyolefin.The optimal amount of HBH polyolefin in the layer depends upon thedegree of branching present in the HBH polymer or polymers, as well ason the nature of any other polymeric component and/or additive in thelayer.

It has been found that the HBH polyolefin utilized in the films of thepresent invention is also capable of forming a hermetic heat seal withother polymers, such as, for example, linear low density polyethylene(LLDPE), very low density polyethylene (VLDPE), ethylene/vinyl acetatecopolymer (EVA), ionomer, and to a lesser extent, nylon, polystyrene,and polyethylene terephthalate.

The second polymer is inclusive of non-highly branched homogeneouscopolymers, as well as highly branched homogeneous polymers having abranching level of less than 40 branches per 1,000 carbon atoms.

As used herein, the term “film” is used in a generic sense to includeplastic web, regardless of whether it is film or sheet. Preferably,films of and used in the present invention have a thickness of 0.25 mmor less. As used herein, the term “package” refers to packagingmaterials configured around (i.e., enveloping) a product being packaged.The phrase “packaged product,” as used herein, refers to the combinationof a product which is surrounded by a packaging material.

As used herein, the phrases “inner layer” and “internal layer” refer toany layer, of a multilayer film, having both of its principal surfacesdirectly adhered to another layer of the film.

As used herein, the phrase “outer layer” refers to any film layer offilm having less than two of its principal surfaces directly adhered toanother layer of the film. The phrase is inclusive of monolayer andmultilayer films. In multilayer films, there are two outer layers, eachof which has a principal surface adhered to only one other layer of themultilayer film. In monolayer films, there is only one layer, which, ofcourse, is an outer layer in that neither of its two principal surfacesare adhered to another layer of the film.

As used herein, the phrase “inside layer” refers to the outer layer of amultilayer film packaging a product, which is closest to the product,relative to the other layers of the multilayer film.

As used herein, the phrase “outside layer” refers to the outer layer, ofa multilayer film packaging a product, which is furthest from theproduct relative to the other layers of the multilayer film. “Outsidelayer” also is used with reference to the outermost layer of a pluralityof concentrically arranged layers simultaneously coextruded through anannular die. Moreover, an outside layer has an “inside surface” and an“outside surface,” the inside surface being that surface of the outsidelayer which is adhered to another film layer, and the outside surface ofthe outside layer being that surface which is not adhered to anotherfilm layer.

As used herein, the phrase “directly adhered”, as applied to filmlayers, is defined as adhesion of the subject film layer to the objectfilm layer, without a tie layer, adhesive, or other layer therebetween.In contrast, as used herein, the word “between”, as applied to a filmlayer expressed as being between two other specified layers, includesboth direct adherence of the subject layer between to the two otherlayers it is between, as well as including a lack of direct adherence toeither or both of the two other layers the subject layer is between,i.e., one or more additional layers can be imposed between the subjectlayer and one or more of the layers the subject layer is between.

As used herein, the phrases “heat-shrinkable,” “heat-shrink” and thelike refer to the tendency of a film, generally an oriented film, toshrink upon the application of heat, i.e., to contract upon beingheated, such that the size (area) of the film decreases while the filmis in an unrestrained state decreases. Likewise, the tension of aheat-shrinkable film increases upon the application of heat if the filmis restrained from shrinking. As a corollary, the phrase“heat-contracted” refers to a heat-shrinkable film, or a portionthereof, which has been exposed to heat such that the film or portionthereof is in a heat-shrunken state, i.e., reduced in size(unrestrained) or under increased tension (restrained).

As used herein, the phrase “free shrink” refers to the percentdimensional change in a 10 cm×10 cm specimen of film, when shrunk at185° F., with the quantitative determination being carried out accordingto ASTM D 2732, as set forth in the 1990 Annual Book of ASTM Standards,Vol. 08.02, pp. 368-371, which is hereby incorporated, in its entirety,by reference thereto. Preferably, the heat shrinkable film has a totalfree shrink (i.e., machine direction plus transverse direction), asmeasured by ASTM D 2732, of at least as 5 percent at 185° C., morepreferably at least 7 percent, still more preferably, at least 10percent, and, yet still more preferably, at least 20 percent, 30percent, 40 percent, 50 percent, 60 percent, 70 percent, and even atleast 80 percent.

As used herein, the phrase “machine direction”, herein abbreviated “MD”,refers to a direction “along the length” of the film, i.e., in thedirection of the film as the film is formed during extrusion and/orcoating. As used herein, the phrase “transverse direction”, hereinabbreviated “TD”, refers to a direction across the film, perpendicularto the machine or longitudinal direction.

As used herein, the term “seal” refers to any seal of a first region ofan outer film surface to a second region of an outer film surface,including heat seals as well as adhesive bonding. In contrast, the term“heat seal” refers to seals made by heating one or more polymericcomponents in one or more films at least to their respective sealinitiation temperatures so that a bond is made upon resolidification ofthe flowing polymer. Sealing can be performed by any one or more of awide variety of manners, such as using a heat seal technique (e.g.,melt-bead sealing, thermal sealing, impulse sealing, dielectric sealing,radio frequency sealing, ultrasonic sealing, hot air, hot wire, infraredradiation, etc.).

A preferred sealing method is impulse heat sealing utilizing seal wireof a material known as Toss Alloy 20, obtained from Toss MachineComponents of Nazareth, Pa. A preferred seal wire is 6 millimeters wideand 0.15 millimeters thick, presenting a flat surface to the film, buthaving tapered edges. In making the seal, the total dwell time is about2 seconds, with the pressure being about 500 pounds across each of theapproximately 23 inch long sealing length.

As used herein, the phrases “seal layer,” “sealing layer,” “heat seallayer,” and “sealant layer,” refer to an outer film layer, or layers,involved in the sealing of the film to itself, another film layer of thesame or another film, and/or another article which is not a film. Itshould also be recognized that in general, up to the outer 3 mils of afilm can be involved in the sealing of the film to itself or anotherlayer. With respect to packages having only fin-type seals, as opposedto lap-type seals, the phrase “sealant layer” generally refers to theinside film layer of a package, as well as supporting layers within 3mils of the inside surface of the sealant layer, the inside layerfrequently also serving as a food contact layer in the packaging offoods. In general, sealant layers employed in the packaging art haveincluded thermoplastic polymers, such as polyolefin (e.g., linear lowdensity polyethylene, very low density polyethylene, homogeneouspolymers such as metallocene catalyzed ethylene/alpha-olefin copolymer,etc), polyamide, polyester (e.g., polyethylene terephthalate glycol),ethylene/ester copolymer (e.g., ethylene/vinyl acetate copolymer),ionomer, etc.

As used herein, the term “bag” is inclusive of L-seal bags, side-sealbags, end-seal bags, backseamed bags, and pouches. An L-seal bag has anopen top, a bottom seal, a seal along a first side edge, and a seamless(i.e., folded, unsealed) second side edge. A side-seal bag has an opentop, a seamless bottom edge, with each of its two side edges having aseal therealong. An end-seal bag is made from a seamless tubing and hasan open top, a bottom seal, and seamless side edges. A pouch has an opentop and a bottom seal and a seal along each side edge. Although sealsalong the side and/or bottom edges can be at the very edge itself,(i.e., seals of a type commonly referred to as “trim seals”), preferablythe seals are spaced inward (preferably ¼ to ½ inch, more or less) fromthe bag side edges, and preferably are made using impulse-type heatsealing apparatus, which utilizes a bar which is quickly heated and thenquickly cooled. A backseamed bag is a bag having an open top, a sealrunning the length of the bag in which the bag film is either fin-sealedor lap-sealed, two seamless side edges, and a bottom seal along a bottomedge of the bag.

As used herein, the phrase “heterogeneous polymer” refers topolymerization reaction products of relatively wide variation inmolecular weight (M_(w)/M_(n) greater than 3.0) and relatively widevariation in composition distribution, i.e., typical polymers prepared,for example, using conventional Ziegler-Natta catalysts. Heterogeneouscopolymers typically contain a relatively wide variety of chain lengthsand comonomer percentages.

As used herein, the phrase “homogeneous polymer” refers topolymerization reaction products of relatively narrow molecular weightdistribution (M_(w)/M_(n) less than 3.0) and relatively narrowcomposition distribution. Homogeneous polymers are useful in variouslayers of the multilayer film used in the present invention. Homogeneouspolymers are structurally different from heterogeneous polymers, in thathomogeneous polymers exhibit a relatively even sequencing of comonomerswithin a chain, a mirroring of sequence distribution in all chains, anda similarity of length of all chains, i.e., a narrower molecular weightdistribution. Furthermore, homogeneous polymers are typically preparedusing metallocene, or other single-site type catalysis, rather thanusing Ziegler Natta catalysts.

More particularly, homogeneous ethylene/alpha-olefin copolymers may becharacterized by one or more processes known to those of skill in theart, such as molecular weight distribution (Mw/Mn), Mz/Mn, compositiondistribution breadth index (CDBI), and narrow melting point range andsingle melt point behavior. The molecular weight distribution(M_(w)/M_(n)), also known as polydispersity, may be determined by gelpermeation chromatography. The homogeneous ethylene/alpha-olefincopolymers useful in this invention generally has (M_(w)/M_(n)) of lessthan 2.7; preferably from about 1.9 to 2.5; more preferably, from about1.9 to 2.3. The composition distribution breadth index (CDBI) of suchhomogeneous ethylene/alpha-olefin copolymers will generally be greaterthan about 70 percent. The CDBI is defined as the weight percent of thecopolymer molecules having a comonomer content within 50 percent (i.e.,plus or minus 50%) of the median total molar comonomer content. The CDBIof linear polyethylene, which does not contain a comonomer, is definedto be 100%. The Composition Distribution Breadth Index (CDBI) isdetermined via the technique of Temperature Rising Elution Fractionation(TREF). CDBI determination clearly distinguishes the homogeneouscopolymers (narrow composition distribution as assessed by CDBI valuesgenerally above 70%) from VLDPEs available commercially which generallyhave a broad composition distribution as assessed by CDBI valuesgenerally less than 55%. The CDBI of a copolymer is readily calculatedfrom data obtained from techniques known in the art, such as, forexample, temperature rising elution fractionation as described, forexample, in Wild et. al., J. Poly. Sci. Poly. Phys. Ed., Vol. 20, p.441(1982). Preferably, homogeneous ethylene/alpha-olefin copolymers have aCDBI greater than about 70%, i.e., a CDBI of from about 70% to 99%. Ahomogeneous ethylene/alpha-olefin copolymer can, in general, be preparedby the copolymerization of ethylene and any one or more alpha-olefin.Preferably, the alpha-olefin is a C₃-C₂₀ alpha-monoolefin, morepreferably, a C₄-C₁₂ alpha-monoolefin, still more preferably, a C₄-C₈alpha-monoolefin. Still more preferably, the alpha-olefin comprises atleast one member selected from the group consisting of butene-1,hexene-1, and octene-1, i.e., 1-butene, 1-hexene, and 1-octene,respectively. Most preferably, the alpha-olefin comprises octene-1,and/or a blend of hexene-1 and butene-1.

Processes for preparing and using homogeneous polymers are disclosed inU.S. Pat. No. 5,206,075, U.S. Pat. No. 5,241,031, and PCT InternationalApplication WO 93/03093, each of which is hereby incorporated byreference thereto, in its entirety. Further details regarding theproduction and use of homogeneous ethylene/alpha-olefin copolymers aredisclosed in PCT International Publication Number WO 90/03414, and PCTInternational Publication Number WO 93/03093, both of which designateExxon Chemical Patents, Inc. as the Applicant, and both of which arehereby incorporated by reference thereto, in their respectiveentireties.

Still another genus of homogeneous ethylene/alpha-olefin copolymers isdisclosed in U.S. Pat. No. 5,272,236, to LAI, et. al., and U.S. Pat. No.5,278,272, to LAI, et. al., both of which are hereby incorporated byreference thereto, in their respective entireties. Each of these patentsdisclose substantially linear homogeneous long chain branchedethylene/alpha-olefin copolymers produced and marketed by The DowChemical Company.

As used herein, the phrase “ethylene/alpha-olefin copolymer”, and“ethylene/alpha-olefin copolymer”, refer to such heterogeneouscopolymers as linear low density polyethylene (LLDPE), and very low andultra low density polyethylene (VLDPE and ULDPE); and homogeneouspolymers such as metallocene catalyzed polymers such as EXACT® resinsobtainable from the Exxon Chemical Company, and TAFMER® resinsobtainable from the Mitsui Petrochemical Corporation. All thesematerials generally include copolymers of ethylene with one or morecomonomers selected from C₄ to C₁₀ alpha-olefin such as butene-1,hexene-1, octene-1, etc. in which the molecules of the copolymerscomprise long chains with relatively few side chain branches orcross-linked structures. This molecular structure is to be contrastedwith conventional low or medium density polyethylenes which are morehighly branched than their respective counterparts. The heterogeneousethylene/alpha-olefins commonly known as LLDPE have a density usually inthe range of from about 0.91 grams per cubic centimeter to about 0.94grams per cubic centimeter. Other ethylene/alpha-olefin copolymers, suchas the long chain branched homogeneous ethylene/alpha-olefin copolymersavailable from the Dow Chemical Company, known as AFFINITY® resins, arealso included as another type of homogeneous ethylene/alpha-olefincopolymer useful in the present invention.

In general, the ethylene/alpha-olefin copolymer comprises a copolymerresulting from the copolymerization of from about 80 to 99 weightpercent ethylene and from 1 to 20 weight percent alpha-olefin.Preferably, the ethylene/alpha-olefin copolymer comprises a copolymerresulting from the copolymerization of from about 85 to 95 weightpercent ethylene and from 5 to 15 weight percent alpha-olefin.

Preferably, the film according to the present invention comprises atotal of from 2 to 20 layers; more preferably, from 2 to 12 layers; morepreferably, from 2 to 9 layers; more preferably, from 3 to 8 layers.Various combinations of layers can be used in the formation of amultilayer film according to the present invention. Given below are someexamples of preferred multilayer film structures in which letters areused to represent film layers (although only 2- through 5-layerembodiments are provided here for illustrative purposes, further layerscould be present):

A/B,

A/C,

A/B/A,

A/B/B′,

A/B/C,

A/B/C/B,

A/B/C/B′,

A/B/C/B/A,

B/A/C/B/A′

wherein

A represents a layer that includes the blend of the highly branchedhomogeneous polymer and the semicrystalline polymer, particularly asemicrystalline ethylene/alpha-olefin copolymer;

B represents a layer including at least one member selected from thegroup consisting of polyolefin (particularly an ethylene/alpha-olefincopolymer), polyester (including polycarbonate), polyamide, polyaromatic(particularly polystyrene and poly(phenol-formaldehyde), andpoly(amine-formaldehyde)), polyether, polyimide, polyimine,polyurethane, polysulfone, polyalkyne; and

C represents a layer including a polymer serving as an oxygen barrierlayer, e.g., polyvinylidene chloride “PVDC” (PVDC homopolymer and/ormethyl acrylate copolymer “PVDC-MA” and/or vinyl chloride copolymer“PVDC-VC”), ethylene/vinyl alcohol copolymer (“EVOH”), polyamide, etc.

Of course, one or more tie layers (“T”) can be used between any one ormore layers of in any of the above multilayer film structures. Also,while “A” represents the above-described blend, “A′” is a differentblend from the A blend, and so on. A film having two “B” layers (asopposed to B and B′) could have the same B polymer(s) or different Bpolymer(s), in the same or different amounts and/or ratios with respectto one another and with respect to the multilayer film as a whole.

As used herein, the phrase “packaging article” is used with reference tobags, pouches, casings, etc. which are useful for the packaging ofproducts.

As used herein, the term “barrier”, and the phrase “barrier layer”, asapplied to films and/or film layers, are used with reference to theability of a film or film layer to serve as a barrier to one or moregases. In the packaging art, oxygen (i.e., gaseous O₂) barrier layershave included, for example, hydrolyzed ethylene/vinyl acetate copolymer(designated by the abbreviations “EVOH” and “HEVA”, and also referred toas “ethylene/vinyl alcohol copolymer”), polyvinylidene chloride(“PVDC”), especially PVDC-methyl acrylate copolymer (“PVDC-MA”), andPVDC-vinyl chloride copolymer (“PVDC-VC”), as well as polyamide,polyester, polyalkylene carbonate, polyacrylonitrile, etc., as known tothose of skill in the art.

Regardless of the structure of the multilayer film of the presentinvention, one or more conventional packaging film additives can beincluded therein. Examples of additives that can be incorporatedinclude, but are not limited to, antiblocking agents, antifoggingagents, slip agents, colorants, flavorings, antimicrobial agents, meatpreservatives, and the like. Where the multilayer film is to beprocessed at high speeds, inclusion of one or more antiblocking agentsin and/or on one or both outer layers of the film structure can bepreferred. Examples of useful antiblocking agents for certainapplications are corn starch and ceramic microspheres.

As is known to those of skill in the art, various polymer modifiers maybe incorporated for the purpose of improving toughness and/ororientability or extensibility of the film. Other modifiers which may beadded include: modifiers which improve low temperature toughness orimpact strength, and modifiers which reduce modulus or stiffness.Exemplary modifiers include: styrene-butadiene, styrene-isoprene, andethylene-propylene.

The films of the present invention can be produced using a hot blownprocess in which the film is extruded through an annular die andimmediately blown by a blown bubble, while the polymer is at or near itsmelt temperature. Such films exhibit a total (i.e., longitudinal plustransverse) free shrink at 185 F. of less than 10 percent, generally nomore than 5 percent in either direction. Such hot blown films are notconsidered to be heat-shrinkable films because the amount ofheat-shrinkability is not high enough to provide the shrink charactertypically required of heat-shrinkable films. Although hot blown filmsare oriented, the orientation occurs in the molten state, withoutproducing the orientation-induced stress which renders the filmheat-shrinkable.

On the other hand, films of the present invention can also be producedusing a cast process. The film can be cast from a slot die with theextrudate being quenched by immediately contacting a chilled roll tocause solidification and cooling, followed by being reheated to atemperature below the melt point (preferably to the softening point ofthe polymer), followed by solid-state orientation using a tenter frame.Alternatively, the film can be formed by downward casting from anannular die, with the resulting annular “tape” being quenched usingcascading water, cooled air (or other gas), or even ambient air. Theresulting solidified and cooled annular tape is then reheated to adesired orientation temperature and oriented while in the solid state,using a trapped bubble.

Films which are oriented in the solid state are considered to beheat-shrinkable, as they have a total free shrink (L+T) at 185° F. ofgreater than 10 percent. Often the quenched extrudate is irradiatedbefore being reheated to the softening temperature of the polymer, i.e.,immediately before the solid-state orientation. The extrudate can evenbe subjected to extrusion coating before reheating. Generally extrusioncoating is carried out after irradiation, to provide a strongersubstrate which is extrusion coated with a polymer sensitive toirradiation, such as PVDC, which is degraded by irradiation. In thismanner, the substrate can be strengthened by irradiative crosslinking inorder to permit a greater degree of solid-state orientation, or simplyto enhance bubble stability during solid-state orientation.

Although the present invention has been described in connection with thepreferred embodiments, it is to be understood that modifications andvariations may be utilized without departing from the principles andscope of the invention, as those skilled in the art will readilyunderstand. Accordingly, such modifications may be practiced within thescope of the following claims.

What is claimed is:
 1. An irradiated multilayer film comprising a firstlayer and a second layer, the first layer being an outer heat seal layercontaining a blend of (A) a highly branched homogeneous polymer havingat least 40 branches per 1000 methylene groups, the highly branchedpolymer having an M_(w)/M_(n) less than 3.0, a density of less than 0.89g/cc, the highly branched homogeneous polymer being present in an amountof from about 5 to 50 weight percent, based on layer weight, and (B) asemicrystalline polyolefin having a density of at least 0.90 g/cc, thesemicrystalline polyolefin being present in an amount of from about 50to 95 weight percent, based on total layer weight, the second layercontaining a thermoplastic polymer in a crosslinked polymer network. 2.The packaging article according to claim 1, wherein the semicrystallinepolyolefin has a density of at least 0.905 g/cc.
 3. The packagingarticle according to claim 1, wherein the highly branched homogeneouspolymer has from 40 to 120 branches per 1000 methylene groups.
 4. Thepackaging article according to claim 1, wherein the semicrystallinepolyolefin comprises at least one member selected from the groupconsisting of very low density polyethylene, linear low densitypolyethylene, low density polyethylene, high density polyethylene,propylene homopolymer, propylene copolymer, linear homogeneousethylene/alpha-olefin copolymer, homogeneous ethylene/alpha-olefincopolymer having long chain branching.
 5. The packaging articleaccording to claim 1, wherein the highly branched homogeneous polymercomprises ethylene homopolymer.
 6. The packaging article according toclaim 1, wherein the multilayer film comprises an oxygen barrier layer.7. The packaging article according to claim 6, wherein the multilayerfilm further comprises a tie layer between the oxygen barrier layer andthe seal layer.
 8. The packaging article according to claim 7, whereinthe heat seal layer is an inside layer of the packaging article, and thetie layer between the oxygen barrier layer and the seal layer is a firsttie layer, the multilayer film further comprising a second tie layerbetween the oxygen barrier layer and an outside layer.
 9. The filmaccording to claim 1, wherein the film is heat-shrinkable.
 10. The filmaccording to claim 9, wherein the film has a total free shrink, at 185°F., of at least 20 percent.
 11. The multilayer film according to claim1, wherein the film has a total free shrink, at 185° F., of less than 10percent.
 12. The film according to claim 1, wherein the film has athickness of from about 0.3-15 mils.
 13. A process for making apackaging article, comprising: (A) extruding a multilayer film having:(i) a first layer which is an outer layer comprising containing a blendof (A) a highly branched homogeneous polymer having at least 40 branchesper 1000 methylene groups, said polymer having an M_(w)/M_(n) less than3.0, a density of less than 0.89 g/cc, the highly branched homogeneouspolymer being present in an amount of from about 5 to 50 weight percent,based on layer weight, and (B) a semicrystalline polyolefin having adensity of at least 0.90 g/cc, the semicrystalline polyolefin beingpresent in an amount of from about 50 to 95 weight percent, based ontotal layer weight; and (ii) a second layer containing a thermoplasticpolymer; and (B) irradiating the multilayer film so that the secondlayer comprises a crosslinked polymer network; and (C) heat sealing theseal layer to itself or another component of the packaging article.